Perfusion of single proximal tubules and peritubular capillaries was used to study the effects of changing intraluminal and intracapillary albumin concentrations on the amount of fluid reabsorbed by the rat proximal tubule. We have been able to achieve similar rates of absolute fluid reabsorption in perfused tubules as in non-perfused ones and conclude that perfusion of tubules and capillaries with artificial solutions does not adversely affect net sodium and fluid transport. Results of our studies show that albumin applied intraluminally has no major effect on fluid reabsorption in perfused tubules. When net sodium movement was prevented by perfusing tubules with Ringer's solution containing 4 mM NaCN, peritubular albumin concentration changes (from 2 to 20g%) had only a minor effect on fluid absorption. However, the effect of peritubular protein concentration changes was dramatically increased when sodium movement was allowed to proceed normally. Hydraulic permeabilities were also measured with a raffinose gradient as a transepithelial driving force and compared to those obtained with albumin as the driving force. The results obtained by these two approaches were in close agreement, the hydraulic permeability being of the order of 8 x 10 to the minus to the 8th power ml.cm super minus 2. sec super minus 1.cm H2O super minus 1. It was concluded from our studies that albumin has only a small direct oncotic effect across the tubular wall. It acts mainly on fluid transport that has occurred secondary to sodium transport by modulating the amount of fluid at a site beyond that of active sodium translocation.